Selective catalytic reduction system

ABSTRACT

A selective catalytic reduction system includes a urea aqueous solution tank to retain a urea aqueous solution to be injected into an exhaust pipe of an engine, a level sensor to detect a solution level height in the tank, a lowpass filter having a signal response characteristic in which a time constant is small when an engine rpm is low and the time constant is larger when the engine rpm is high, with respect to an output signal of the sensor, a remaining amount indicator to indicate a remaining amount of the solution based on an output signal of the sensor processed by the lowpass filter, and a unit to prohibit the engine from being started when the remaining amount of the solution is less than a lower limit value.

TECHNICAL FIELD

The present invention relates to an SCR system that purifies exhaust gasby injecting a urea aqueous solution to the exhaust gas of a dieselvehicle, and more particularly, to an SCR system that can rapidly cancelengine start prohibition after filling the urea aqueous solution as wellas preventing improper indication of deficiency of the urea aqueoussolution by variation in solution level in a urea aqueous solution tankand unwanted engine start prohibition.

BACKGROUND ART

As an exhaust gas purifying system for purifying NOx in the exhaust gasof a diesel engine, an SCR system using a SCR (selective catalyticreduction) apparatus has been developed.

The SCR system supplies the urea aqueous solution to upstream of theexhaust gas of the SCR apparatus, generates ammonia by heat of theexhaust gas, and reduces and purifies NOx on an SCR catalyst by theammonia (see, for example, Patent Document 1).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Publication No.2000-303826

DISCLOSURE OF THE INVENTION Problems To Be Solved By The Invention

A sufficient amount of urea aqueous solution needs to be retained inorder for a vehicle to travel while continuously purifying the exhaustgas, and as a result, the urea aqueous solution tank is provided. Alevel sensor that detects a remaining amount of the urea aqueoussolution is provided in the urea aqueous solution tank, and theremaining amount is indicated by meters. When the remaining amount isless than a lowerlimit value, the deficiency of the urea aqueoussolution is indicated to warn a driver and prohibit the engine frombeing started, thereby preventing travelling in a state where theexhaust gas cannot be purified.

However, the level sensor is a sensor that detects the position of afloat that floats on the solution level of the urea aqueous solution asa solution level height. Therefore, when vibration is generated in thevehicle, the solution level vibrates, and as a result, the detectedremaining amount varies. A warning is given due to the variation in thesolution level caused by the vibration of the vehicle or the enginecannot be started.

In order to solve the problems, for example, it is considered that anoutput signal of the level sensor is input through a lowpass filter.However, in order to remove a vibration component of the solution levelfrom the output signal of the level sensor, a time constant of thelowpass filter may be several seconds. Therefore, the indication of theremaining amount on an actual solution level is followed with delay. Asa result, for example, when the driver who verifies that the indicationof the remaining amount indicates the deficiency of the urea aqueoussolution fills the urea aqueous solution tank with the urea aqueoussolution and thereafter, attempts starting the engine without delay, theindication of the remaining amount has yet indicated the deficiency ofthe urea aqueous solution, and as a result, there is a case in which theengine is not started. Although the urea aqueous solution is filled up,when the deficiency of the urea aqueous solution is indicated or theengine is not started, the driver feels a sense of strangeness. Further,when the urea aqueous solution is being filled up, if the indication ofthe remaining amount is not increased together, it may be misunderstoodthat the level sensor breaks down.

There is a case in which the remaining amount is also estimated inparallel by adding up an injection amount of the urea aqueous solutionin addition to detecting an actual remaining amount of the urea aqueoussolution tank by the level sensor. In this case, when the indication ofthe remaining amount by the level sensor rises, adding-up is initializedby misjudgment that the urea aqueous solution is filled up. Further,since the estimated remaining amount based on the adding-up of theinjection amount of the urea aqueous solution is unilaterally reducedwhile the remaining amount detected by the level sensor varies, adifference between both parts occurs and management of the remainingamount becomes uncertain.

Accordingly, an object of the present invention is to solve theproblems, and provide an SCR system that can rapidly cancel engine startprohibition after filling the urea aqueous solution as well aspreventing improper indication of deficiency of the urea aqueoussolution by variation in a solution level in a urea aqueous solutiontank and unwanted engine start prohibition.

Means for Solving the Problems

To achieve the object described above, an SCR system according to thepresent invention includes: a urea aqueous solution tank configured toretain a urea aqueous solution to be injected into an exhaust pipe of anengine; a level sensor configured to detect a solution level height inthe urea aqueous solution tank; a lowpass filter having a signalresponse characteristic in which a time constant is small when an enginerpm is low and the time constant is larger when the engine rpm is high,with respect to an output signal of the level sensor; a remaining amountindicator configured to indicate a remaining amount of the urea aqueoussolution based on an output signal of the level sensor processed by thelowpass filter; and a start prohibiting unit in deficiency of the ureaaqueous solution configured to prohibit the engine from being startedwhen the remaining amount of the urea aqueous solution is less than alowerlimit value.

The time constant of the lowpass filter is 0 in the range of the enginerpm of 0 to a cranking rpm and may increase with the increase in theengine rpm over the cranking rpm.

Effects of the Invention

The present invention provides excellent effects as follows.

(1) Improper indication of deficiency of the urea aqueous solution byvariation in a solution level in a urea aqueous solution tank andunwanted engine start prohibition are prevented.

(2) Engine start prohibition can be rapidly cancelled after filling theurea aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of principal components of an SCRsystem according to an embodiment of the present invention.

FIG. 2 is a configuration diagram, in detail, illustrating the SCRsystem according to the embodiment of the present invention.

FIG. 3 is an input/output configuration diagram of the SCR system ofFIG. 1

FIG. 4 is a flowchart illustrating a sequence of filter processingaccording to the present invention.

FIG. 5 is a filter characteristic diagram of a lowpass filter used inthe present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings.

As illustrated in FIGS. 1 and 2, an SCR system 100 according to thepresent invention includes a urea aqueous solution tank 105 configuredto retain a urea aqueous solution to be injected into an exhaust pipe102 of an engine E, a level sensor 120 configured to detect a solutionlevel height in the urea aqueous solution tank 105, a lowpass filter 1having a signal response characteristic in which a time constant issmall when an engine rpm is low and the time constant is larger when theengine rpm is high with respect to an output signal of the level sensor120, a remaining amount indicator 2 configured to indicate a remainingamount of the urea aqueous solution based on an output signal of thelevel sensor 120 processed by the lowpass filter 1, and a startprohibiting unit 3 in deficiency of the urea aqueous solution configuredto prohibit the engine from being started when the remaining amount ofthe urea aqueous solution is less than a lowerlimit value.

The remaining amount indicator 2 is configured by a liquid crystal orlight emitting diode, and is installed in a cabin of a vehicle, forexample, a console panel. An indication content of the remaining amountindicator 2 is edited by a remaining amount indication control unit 4.The remaining amount indication control unit 4 outputs a remainingamount by a pointer, a bar graph, and the like, and a warning of thedeficiency of the urea aqueous solution to the remaining amountindicator 2.

The lowpass filter 1 is configured by a digital filter. The timeconstant of the lowpass filter 1 is set to 0 in the range of the enginerpm of 0 to a cranking rpm and is set to increase with the increase inthe engine rpm over the cranking rpm. In detail, a filter coefficient togive the time constant for each engine rpm is set in a time constant map5 referred to as the engine rpm. The filter coefficient is provided inthe digital filter, and as a result, a signal response characteristic ofthe lowpass filter 1 is determined.

The level sensor 120 includes a scale 6 that stands in the urea aqueoussolution tank 105, a plurality of approximate sensors (not illustrated)arranged in parallel vertically in the scale 6, and a float 7 thatfloats on a solution level S of the urea aqueous solution and is movablewith the scale 6, and is configured to detect the position of theapproximate sensor that detects the float 7 as the height of thesolution level S.

In detail, as illustrated in FIG. 2, the SCR system 100 primarilyincludes an SCR apparatus 103 provided in the exhaust pipe 102 of theengine E, a dosing valve (a urea aqueous solution injecting device, adosing module) 104 configured to inject the urea aqueous solutionupstream of the SCR apparatus 103 (the exhaust gas), a urea aqueoussolution tank 105 configured to retain the urea aqueous solution, asupply module 106 configured to supply the urea aqueous solutionretained in the urea aqueous solution tank 105 to the dosing valve 104,and a DCU (dosing control unit) 126 configured to control the dosingvalve 104 or the supply module 106.

The lowpass filter 1, the start prohibiting unit 3 in deficiency of theurea aqueous solution, the remaining amount indication control unit 4,and the time constant map 5 may be provided in the DCU 126.

In the exhaust pipe 102 of the engine E, a DOC (diesel oxidationcatalyst) 107, a DPF (diesel particulate filter) 108, and the SCRapparatus 103 are sequentially arranged from upstream to downstream ofthe exhaust gas. The DOC 107 oxidizes NO in the exhaust gas exhaustedfrom the engine E to form NO₂ and is used to improve denitrationefficiency in the SCR apparatus 103 by controlling a ratio of NO and NO₂in the exhaust gas. Further, the DPF 108 is used to collect PM(particulate matter) in the exhaust gas.

The dosing valve 104 is provided in the exhaust pipe 102 upstream of theSCR apparatus 103. The dosing valve 104 has a structure in which aninjection hole is provided in a cylinder filled with high-pressure ureaaqueous solutions and a valve body clogging the injection hole isattached to a plunger, and is configured to inject the urea aqueoussolution by making the valve body be spaced apart from the injectionhole by raising the plunger through electrical conduction to a coil.When the electrical conduction to the coil stops, the plunger is droppedby internal elastic force, and as a result, the valve body clogs theinjection hole, thereby stopping the injection of the urea aqueoussolution.

An exhaust temperature sensor 109 configured to measure the temperatureof the exhaust gas at an inlet of the SCR apparatus 103 (the temperatureof an inlet of the SCR) is provided in the exhaust pipe 102 upstream ofthe dosing valve 104. Further, an upstream NOx sensor 110 configured todetect a NOx concentration upstream of the SCR apparatus 103 is providedupstream of the SCR apparatus 103 (herein, upstream of the exhausttemperature sensor 109), and a downstream NOx sensor 111 configured todetect the NOx concentration downstream of the SCR apparatus 103 isprovided downstream of the SCR apparatus 103.

The supply module 106 includes an SM pump 112 configured to pump theurea aqueous solution, an SM temperature sensor 113 configured tomeasure the temperature of the supply module 106 (the temperature of theurea aqueous solution that flows in the supply module 106), a ureaaqueous solution pressure sensor 114 configured to measure the pressureof the urea aqueous solution in the supply module 106 (the pressure at adischarge side of the SM pump 112), and a reverting valve 115 configuredto switch supplying the urea aqueous solution from the urea aqueoussolution tank 105 to the dosing valve 104 or reverting the urea aqueoussolution in the dosing valve 104 to the urea aqueous solution tank 105by switching a path of the urea aqueous solution. Herein, when thereverting valve 115 is in an off state, the urea aqueous solution fromthe urea aqueous solution tank 105 is configured to be supplied to thedosing valve 104 and when the reverting valve 115 is in an on state, theurea aqueous solution in the dosing valve 104 is configured to bereverted to the urea aqueous solution tank 105.

When the reverting valve 115 is switched to supply the urea aqueoussolution to the dosing valve 104, the supply module 106 is configured tofeed the urea aqueous solution in the urea aqueous solution tank 105 andsuction the fed urea aqueous solution through a solution sending line(suction line) 116, in the SM pump 112, supply the suctioned ureaaqueous solution to the dosing valve 104 through a pumping line(pressure line) 117, and revert a remnant urea aqueous solution to theurea aqueous solution tank 105 through a recovery line (back line) 118.

An SCR sensor 119 is provided in the urea aqueous solution tank 105. TheSCR sensor 119 includes a level sensor 120 configured to measure theheight (level) of the solution level of the urea aqueous solution in theurea aqueous solution tank 105, a temperature sensor 121 configured tomeasure the temperature of the urea aqueous solution in the urea aqueoussolution tank 105, and a quality sensor 122 configured to measure thequality of the urea aqueous solution in the urea aqueous solution tank105. The quality sensor 122 detects the concentration of the ureaaqueous solution or whether a heterogeneous mixture is mixed in the ureaaqueous solution, from, for example, a propagation velocity ofultrasonic waves or electrical conductivity, and detects the quality ofthe urea aqueous solution in the urea aqueous solution tank 105.

A cooling line 123 configured to circulate cooling water for cooling theengine E is connected to the urea aqueous solution tank 105 and thesupply module 106. The cooling line 123 passes through the urea aqueoussolution tank 105 to exchange heat between the cooling water that flowsin the cooling line 123 and the urea aqueous solution in the ureaaqueous solution tank 105. Similarly, the cooling line 123 passesthrough the supply module 106 to exchange heat between the cooling waterthat flows in the cooling line 123 and the urea aqueous solution in thesupply module 106.

A tank heater valve (coolant valve) 124 configured to switch the supplyof the cooling water to the urea aqueous solution tank 105 and thesupply module 106 is provided in the cooling line 123. Further, thecooling line 123 is connected to even the dosing valve 104, but thecooling water is configured to be supplied to the dosing valve 104regardless of opening/closing of the tank heater valve 124. Although notillustrated due to simplification of the drawing in FIG. 2, the coolingline 123 is installed along the solution sending line 116, the pumpingline 117, and the recovery line 118 through which the urea aqueoussolution passes.

FIG. 3 illustrates an input/output configuration diagram of the DCU 126.

As illustrated in FIG. 3, input signal lines from the upstream NOxsensor 110, the downstream NOx sensor 111, the SCR sensor 119 (the levelsensor 120, the temperature sensor 121, and the quality sensor 122), theexhaust temperature sensor 109, the SM temperature sensor 113 and theurea aqueous solution pressure sensor 114 of the supply module 106, andan ECM (engine control module) 125 configured to control the engine Eare connected to the DCU 126. Signals of an outdoor temperature andengine parameters (engine rpm, and the like) are input from the ECM 125.

An instruction of the engine start prohibition determined by the startprohibiting unit 3 in deficiency of the urea aqueous solution, theremaining amount, and the warning are output from the DCU 126 to the ECM125. Information on the remaining amount and the warning is sent fromthe ECM 125 to the remaining amount indicator 2.

Output signal lines to the tank heater valve 124, the SM pump 112 andthe reverting valve 115 of the supply module 106, the dosing valve 104,a heater of the upstream NOx sensor 110, and a heater of the downstreamNOx sensor 111 are connected to the DCU 126. An input/output of signalsbetween the DCU 126 and each member may be an input/output throughindividual signal lines or an input/output through a CAN (controllerarea network).

The DCU 126 is configured to determine the amount of the urea aqueoussolution injected from the dosing valve 104 based on the estimatedamount of NOx in the exhaust gas as well as estimating the amount of NOxin the exhaust gas based on signals of the engine parameters from theECM 125 and the temperature of the exhaust gas from the exhausttemperature sensor 109, and further, control the dosing valve 104 basedon a detection value of the upstream NOx sensor 110 and adjust theamount of the urea aqueous solution injected from the dosing valve 104when the urea aqueous solution is injected at the amount of the ureaaqueous solution determined in the dosing valve 104.

Hereinafter, an operation of the SCR system 100 according to the presentinvention will be described.

As illustrated in FIG. 4, in step S41, the lowpass filter 1 refers tothe time constant map 5 as the engine rpm and sets the filtercoefficient. Subsequently, in step S42, the lowpass filter 1 filters theoutput signal of the level sensor 120 to obtain the remaining amount ofthe urea aqueous solution.

Subsequently, in step S43, the start prohibiting unit 3 in deficiency ofthe urea aqueous solution determines whether the remaining amount of theurea aqueous solution is less than the lowerlimit value. In the case ofYES, the process proceeds to step S44. In the case of NO, the processproceeds to step S45. In step S44, the start prohibiting unit 3 indeficiency of the urea aqueous solution prohibits the engine E frombeing started because the remaining amount of the urea aqueous solutionis less than the lowerlimit value. Meanwhile, in step S45, the startprohibiting unit 3 in deficiency of the urea aqueous solution permitsthe start of the engine because the remaining amount of the urea aqueoussolution is more than the lowerlimit value.

According to the above sequence, the output signal of the level sensor120 is filtered to become the remaining amount of the urea aqueoussolution and when the remaining amount of the urea aqueous solution isless than the lowerlimit value, the engine E is prohibited from beingstarted. Further, although not illustrated in the sequence, theremaining amount of the urea aqueous solution is indicated by theremaining amount indicator 2 and when the remaining amount of the ureaaqueous solution is less than the lowerlimit value, a warning regardingthe deficiency of the urea aqueous solution is indicated in theremaining amount indicator 2.

Herein, a filter characteristic of the lowpass filter 1 given by thetime constant map 5 and an effect accompanied thereby will be describedwith reference to FIG. 5.

The time constant of the lowpass filter 1 is 0 in the range of theengine rpm of 0 to the cranking rpm. This is equivalent to a case inwhich no filter is present and the output signal of the level sensor 120becomes the remaining amount of the urea aqueous solution as it is.Therefore, the indication of the remaining amount on an actual solutionlevel is followed with delay. Accordingly, when the urea aqueoussolution is being filled up, the remaining amount indication isincreased together, and as a result, it is not misunderstood that thelevel sensor 120 breaks down. Further, when the remaining amountindication indicates the deficiency of the urea aqueous solution, theurea aqueous solution is filled in the urea aqueous solution tank 105 tosolve the deficiency of the urea aqueous solution without delay, and asa result, the engine start is enabled.

The time constant is increased with the increase in the engine rpm inthe range of the cranking rpm to an idle rpm. For example, the timeconstant in the idle rpm is several seconds. The time constant is gentlyincreased in the idle rpm or more as compared with the range of thecranking rpm to the idle rpm. When the time constant is small around thecranking rpm, so called the filter coefficient is light and aninterruption frequency is high. When the time constant is increased, socalled the filter coefficient is heavy and the interruption frequency islowered. As such, since a high-frequency interruption effect by thelowpass filter 1 is shown in the cranking rpm or more, or a situation inwhich the warning is given or the engine is disabled to be started isprevented, due to the variation in the solution level caused by thevibration of the vehicle. Therefore, for example, when the vehicle stopstraveling to be in an idle operation, an influence by the vibration ofthe engine is removed. Further, while the vehicle travels, an influenceof vibration of a vehicle body by the travelling is also removed.

As described above, according to the SCR system 100 of the presentinvention, since the output signal of the level sensor 120 is processedto be the remaining amount of the urea aqueous solution by using thelowpass filter 1 having the signal response characteristic in which thetime constant is small when the engine rpm is low and the time constantis large when the engine rpm is high, improper indication of deficiencyof the urea aqueous solution by variation in a solution level in theurea aqueous solution tank 105 and unwanted engine start prohibition canbe prevented, and further, engine start prohibition after filling theurea aqueous solution can be rapidly cancelled. When the urea aqueoussolution is being filled up, the remaining amount indication isincreased together, and as a result, it is not misunderstood that thelevel sensor 120 breaks down.

EXPLANATION OF REFERENCE NUMERALS

1 lowpass filter

2 remaining amount indicator

3 start prohibiting unit in deficiency of urea aqueous solution

4 remaining amount indication control unit

5 time constant map

6 scale

7 float

100 SCR system

1. An SCR system, comprising: a urea aqueous solution tank configured toretain a urea aqueous solution to be injected into an exhaust pipe of anengine; a level sensor configured to detect a solution level height inthe urea aqueous solution tank; a lowpass filter having a signalresponse characteristic in which a time constant is small when an enginerpm is low and the time constant is larger when the engine rpm is high,with respect to an output signal of the level sensor; a remaining amountindicator configured to indicate a remaining amount of the urea aqueoussolution based on an output signal of the level sensor processed by thelowpass filter; and a start prohibiting unit in deficiency of the ureaaqueous solution configured to prohibit the engine from being startedwhen the remaining amount of the urea aqueous solution is less than alowerlimit value.
 2. The SCR system according to claim 1, wherein thetime constant of the lowpass filter is 0 in the range of the engine rpmof 0 to a cranking rpm and increases with the increase in the engine rpmover the cranking rpm.